Life history traits and the population dynamics of a species may vary considerably across space and over time. All to often biologists have viewed variability as noise interfering with the precision of parameter estimates, rather than as a source of information about the population of interest. However, understanding the influence of environmental (spatial and temporal) variation on demographic parameters is essential to understanding the population dynamics of a given species (Ricklefs 1973, Rhodes and Odum 1996). Recognition of information needs for management decisions and conservation strategies (Rhodes and Odum 1996) has resulted in an increased emphasis on correlations to spatial and temporal environmental variation in relation to demographic studies (Ricklefs 1973, Stearns 1992).

The snail kite (Rostrhamus sociabilis) is an endangered raptor whose distribution in the United States is restricted to the wetlands of South Florida Ecosystem, including watersheds of the Everglades, Lake Okeechobee, Kissimmee River, and Upper St. Johns River. In addition to being endangered, it is an obligate wetland dependent species feeding almost exclusively on one species of aquatic snail, the apple snail (Pomacea paludosa). The viability of the kite in the United States is therefore dependent on the hydrologic conditions (both long and short-term) appropriate for maintaining a mosaic of both its forage and nesting habitats across the region.

There have been several attempts at modeling snail kite populations. Nichols et al. (1980) developed deterministic and stochastic models in an attempt to better understand the life history of snail kites, and more recently Beissinger (1995) developed a population viability model based on "environmental states". As Beissinger (1995) pointed out, a weakness of his model was the lack of a spatial component. Currently, Dr. Wolf Mooij of the Nederlands Institute of Ecology in colloaboration with D. De Angelis, R. Bennetts, and W. Kitchens, is developing a individual-based model on snail kites in Florida as part of the ongoing restoration efforts in central and southern Florida. The model gives an individual-based, spatially explicit description of the dynamics of the snail kite population in relation to the hydrological conditions of central and southern Florida. Providing statistically valid demographic parameter estimates has been critical for development and remains critical for the application of the individual-based model. Given that snail kites are dietary specialists on the apple snail it is critical to link kite demography to the dynamics of apple snails. Currently snail dynamics as related to complex hydrologic changes are being studied and modeled (Dr. Phil Darby, University of West Florida). Incorporating both the dynamics of the apple snails, the habitat conditions among the various units comprising the South Florida hydroscape, and current and planned hydrologic regimes as sources of demographic influences is critical to the on-going modelling effort.

Purpose/Relevance to the South Florida Ecosystem:

This study will provide reliable estimates of critical temporal and spatial factors structuring the demographic responses of snail kites as well as to provide a sound monitoring program during implementation of restoration.

Study Design:

Mark-resighting methods have a long and solid statistical foundation for estimating survival and population size (Lebreton et al. 1992; Nichols 1992). And more recently, mark-resighting methods have been developed to estimate recruitment and the rate of population change (Pradel 1996, Nichols et al. In press). In 1992, a mark-resighting program was initiated to estimate survival of snail kite (Bennetts and Kitchens 1997). More recently, this approach has been used to estimate the population size and rate of population change in snail kites (Dreitz 2000). Our study design would continue the use of mark-resighting techniques to monitor the snail kite population. Our approach would also attempt to incorporate spatial and temporal components of environmental variation (i.e., habitat conditions in wetland units, hydrological dynamics, apple snail densities) to assess how these factors influence the demographic parameters (i.e., survival and reproduction). The most efficacious approach, manipulative experiments (Harrison and Cappucino 1995) are impractical for threatened and endangered species. We believe that our proposed approach employing observational studies that concentrate on valid parameter estimation, model selection, exploration of process variation (i.e., understanding the components of variance estimates), and examination of different factors under varying environmental conditions is the only feasible alternative.

Expected Benefits:

It is important to recognize that for the snail kite, the argument as to whether a single general factor, such as some hydrological variable, regulates or limits the population becomes problematical when interactions are considered. The influence of hydrology on the population dynamics of the snail kite varies depending on changes in other factors (i.e., food resources, vegetation structure, climate). We believe that our proposed approach will aid in statistically resolving the magnitude and relative importance of these often closely correlated factors. This will then allow refinement to the management models providing critical linkage to the design and evaluation of restoration alternatives. In addition these studies will additionally provide long-term observations that are directly translatable to snail kite monitoring needs.

* Note: Manuscripts, peer reviewed publications, book chapters, graduate student thesis/dissertation, etc. are both acceptable and desirable as chapters or sections of annual/final reports. At least one 2-page fact sheet is required for each of these major types of publications.